Reflective diffraction gratings are used to provide wavelength dispersion in wavelength-selective optical devices, such as wavelength selective switches (WSSs). Conventional reflective diffraction gratings include metal-coated diffraction gratings and multilayer dielectric gratings.
Although many fabrication methods exist, metal-coated diffraction gratings are most often produced by replication. In this fabrication method, a ruled or holographically patterned master is embossed or copied into a resin grating layer, e.g., formed of epoxy, to form a surface-relief pattern therein. The surface-relief pattern is then coated with a conforming reflective metal layer to form the metal-coated diffraction grating. Unfortunately, the resin grating layer is not stable at high temperatures, e.g., greater than 130° C. for an epoxy grating layer. Therefore, care must be taken to use low temperatures during the manufacture of devices incorporating such metal-coated diffraction gratings in order to prevent temperature-induced changes in the surface-relief pattern. Furthermore, any demolding materials or release layers used during the fabrication of such metal-coated diffraction gratings must be entirely removed from the resin grating layer prior to coating with the reflective metal layer in order to avoid reliability issues.
On the other hand, multilayer dielectric gratings are, typically, produced by direct etching. In this method, a surface-relief pattern is etched through an etch mask into a reflective thin-film stack, which includes a large number of thin-film dielectric layers. For example, multilayer dielectric gratings including a reflective thin-film stack formed of alternating silica (SiO2) and tantala (Ta2O5) layers are disclosed in U.S. Pat. No. 8,238,025 to Parriaux, issued on Aug. 7, 2012, in U.S. Pat. No. 6,680,799 to Parriaux et al., issued on Jan. 20, 2004, in U.S. Pat. No. 5,907,436 to Perry et al., issued on May 25, 1999, and in U.S. Patent Application Publication No. 2008/0062523 to Rancourt, published on Mar. 13, 2008, which are incorporated herein by reference. Unfortunately, such multilayer dielectric gratings are, generally, difficult and expensive to manufacture because of the large number of process steps involved. Furthermore, when the etch mask is patterned by holography at a recording wavelength, reflection of light at the recording wavelength by the reflective thin-film stack and/or a substrate on which the thin-film stack is disposed may lead to undesired interference patterns detrimental to the resulting surface-relief pattern.